There are two methods utilizing liquid crystals in light shutters for alpha-numeric displays and the like. One of these uses a light scattering effect generally known as dynamic scattering; while the other is of the field effect type employing a twisted nematic structure. The present invention is particularly adapted for use in field effect liquid crystal light shutter displays such as those shown in U.S. Pat. No. 3,731,986, issued May 8, 1973. Light shutters of this type comprise a layer of nematic liquid crystal material sandwiched between a pair of parallel transparent plates, the plates being coated on selected areas thereof with transparent conducting material to form an alpha-numeric display. The surfaces of the plates in contact with the liquid crystal material are rubbed at right angles to each other to effect a twisted nematic structure. Application of an electrical potential between the transparent electrodes will cause the nematic structure to rotate or untwist. By providing polarizers on opposite sides of the display, polarized light can be made to pass through the structure or be blocked, depending upon whether the polarizers are crossed or parallel to each other.
The liquid crystal material used in a light shutter, whether of the dynamic scattering or field effect type, is very sensitive to moisture contained in the air. This sensitivity is exhibited by an increase in the impressed voltage required to cause the crystals to rotate or untwist when an electrical field is applied thereacross. Two techniques have been attempted in the past to isolate these crystals from contaminants. In one technique, the liquid crystal material is hermetically sealed between glass plates with a ceramic or glass seal. In the other technique, a thermosetting silicone type sealer is utilized as a gasket around the layer of liquid crystal material, followed by a thermosetting epoxy adhesive to hold the glass plates together at a constant distance.
Neither of the foregoing methods for sealing a layer of liquid crystal material between parallel glass plates has proven wholly satisfactory. The ceramic and glass seals are expensive and require the addition of the liquid crystal to the space between the plates through one of two small holes drilled into the surface of one of the glass plates followed by sealing the holes. This is altogether a slow and expensive operation and not well suited to thin cells. In addition, the heat associated with this method of sealing may destroy the surface alignment required for operation of field effect liquid crystal displays of the type shown, for example, in the foregoing U.S. Pat. No. 3,731,986. The silicon-epoxy sealing system, while easier to manufacture, has not provided an adequate seal. With it, the current usage keeps increasing as contaminants are absorbed through the seal from the atmosphere.